The most common means for propelling ships is the screw propeller wherein the axis of rotation of the blades is disposed along the direction of movement of the ship. To reduce fuel consumption, the efficiency of the propeller should be as high as possible. In this context, the efficiency of a propeller that is mounted on a ship is defined as the ratio between the power needed to propel the ship forward and the power needed to simply drag the ship forward. Typically, the efficiency of a propeller is 60-70%. Since fuel consumption is directly dependent on the efficiency of the propeller, any improvement in the efficiency results in a corresponding reduction of the fuel consumption.
In order to improve the efficiency of propellers, it has been suggested that the propeller be combined with a streamlined body arranged behind the propeller and coaxial with the propeller. Such a streamlined body is sometimes referred to as a Costa-bulb, propulsion bulb or simply bulb. Such a propulsion bulb is disclosed in, for example, British patent specification GB 762,445. That document discloses an arrangement where a propeller is mounted on a ship in front of a rudder having a rudder post. A bulb is placed behind the propeller and a supporting member for the bulb is formed by the rudder post. It has also been suggested in WO 97/11878 that a torpedo-shaped body can be placed behind the propeller. The torpedo-shaped body is described as being suspended in the rudder horn and unable to be swung relative to the ship.
For a ship, it is also desirable that the manoeuvrability is as good as possible. In this context, manoeuvrability is defined as the side force that can be accomplished with a certain angular displacement of the rudder.
It is an object of the present invention to provide an arrangement for steering and propulsion of a ship which has an improved efficiency. It is a further object of the invention to provide an arrangement for steering and propulsion that has an improved manoeuvrability without increased steering gear torque.